1. Field of the Invention
The invention relates to a process for removing undesirable constituents from a gas by heterogeneous catalysis.
2. Description of the Prior Art
The catalytic combustion of carrier-fixed contaminants is known in principle and can be carried out using a wide variety of catalysts, as is disclosed in DE A 38 04 722. The disadvantage of the previously known catalytic processes lies in the fact that all of the carrier air must be heated to the working temperature of the catalytic material (200 to 600 degrees Celsius). During combustion processes, e.g., in vehicle engines, the exhaust gas to be purified is itself already at the required process temperature. In the former case, in which the required energy must be supplied from the outside, this entails high energy expenditure and limits the application area of catalytic post-combustion.
To overcome this problem, it has already been suggested, for example in DE A 3929521 that only the catalytic surface be brought to the required working temperature, so that a carrier air of approximately room temperature can be worked with. For this, a heating wire coated with a catalytic material is used that is directly heated through the application of an electric voltage. The carrier-fixed contaminated air is then conducted, for example, through a tight screen of catalytic wires mounted to a frame made of an electrically insulating material, so quickly that the air heats up slightly and the contaminants are largely broken down by catalytic oxidation. A disadvantage of this process is the poor efficiency of the unit, since every air molecule must be brought into direct contact with the coated heating wires. This is not practically possible at the throughput of contaminated exhaust air required for the performance of the unit. The process described does not work in the case of exhaust air with a low concentration of contaminants, especially when the level falls below 100 mg/m.sup.3 of exhaust air.
Along with the process of direct catalytic combustion, the process of prior adsorption of contaminants from carrier-fixed exhaust air by means of an adsorbate is known from DE A 30 21 174. In this generic process, the gas to be purified is conducted at a temperature of between 0.degree. and 250.degree. C. by means of pressure through a bed of a particulate adsorber that consists of a porous ceramic carrier with a large specific inner surface and is impregnated with a material active at elevated temperature as an oxidation catalyst. First the gaseous constituents are adsorbed and, no later than after the bed is loaded with the maximum load capacity theoretically possible, the bed is heated to a process temperature between 250 and 350.degree. C. and the process temperature is maintained over a set time interval and, after the process temperature is reached, the constituents are catalytically combusted at the adsorption site. After the heating is terminated and the bed cools to the operating temperature, adsorptive addition resumes. The heating of the bed occurs by virtue of a burner being located in the feed pipe to the reaction chamber. This burner is ignited when the adsorbent is to be regenerated. A disadvantage in this procedure is the indirect heating of the adsorber by the heated gas to be purified, since heat transmission from a gas to a solid is poor. As a result, the solid reaches the required process temperature very slowly, and catalytic combustion does not take place or takes place only incompletely during this heating phase. Because of this, post-purification is necessary to surmount this phase of incomplete combustion. However, the installation of post-purification equipment entails a considerable financial expenditure and also means that energy will be lost. Another disadvantage of the suggested process is the fact that temperatures of more than 800.degree. C. can be reached in the adsorbent during regeneration. At these high temperatures, the danger exists that the carrier material will bake through sintering or that the catalytic effect will be destroyed.